I don't think it's too bad. I been lugging one on and off various services regularly for a few years now, and I've only not got a bike on a train twice - once Virgin been jobsworths regarding a reservation at Birmingham...the other time, on Arriva North Wales during the hols, as the train was too packed for most humans to get on, nm bikes.
If you book, generally you don't have problems (East Coast Online, I never phone). If you don't, mostly you can wing it on the long-distance trains (i.e. they don't ask, or just say you have a reservation - I've never been asked to produce a reservation). On local trains 9/10 you don't need a reservation, but you may be fighting for bike space with other cyclists.
I have found Virgin to be the worst sticklers by far for reservations. To be fair to them, if you book, your fine. And they do have to have crew available in advance to let you off and open up the bike coach - either on the platform, or the ticket collector has to walk up the train and get in position. This goes for most of the long-distance trains afaik. Yes I know it sucks if you get refused as half the time nobody is occupying the bike spaces.